The present invention relates to a crack opening displacement gage which is able to operate at elevated temperatures and has high resolution and accuracy. The device of this invention may be used in many different applications such as in the testing of fracture toughness specimens and in measuring displacement in structural joints.
There are a number of different types of devices that are presently in use for measuring crack opening displacements at moderate temperatures. The most popular type of device currently in use employs bonded resistance strain gages mounted on cantilever beams that undergo changes in strain as the crack opening changes. These devices are limited in use to a maximum temperature of about 600.degree. F. by the fundamental characteristics of the strain gages. Crack opening displacement gages that are based on variable reluctance principles suffer to a similar degree when operated at elevated temperatures. Techniques that use deflection transmitting rods to transfer displacements from an elevated temperature environment to a more benign environment are quite cumbersome and are often unreliable and inaccurate.
The patent literature also shows some crack opening displacement devices. See for example, U.S. pat. Nos. 3,000,101; 2,732,625; 2,921,282 and 3,566,222. The devices shown in these prior art patents generally are not characterized by good resolution and accuracy especially in comparison with the device of the present invention and these prior art devices are not constructed for operation at elevated temperatures. These devices generally rely upon a variation in a gap between a pair of capacitor plates which has been found to be not as accurate a form of sensing as the sensing arrangement of the present invention. For one thing there may be many other factors that effect the gap-changing capacitive displacement sensor such as gap changes due to vibrations or temperature fluctuations. The structure of the present invention inherently compensates for these factors.
Accordingly, one object of the present invention is to provide an improved crack opening displacement gage employing an improved differential capacitive displacement sensing arrangement.
Another object of the present invention is to provide a crack opening displacement transducer that is capable of operation at elevated temperatures of even up to and exceeding 1500.degree. F.
A further object of the present invention is to provide an electrical transducer for measuring crack opening displacements that is portable, easily attachable within an existing crack opening and reuseable even after a catastrophic failure has occurred at the crack.
Still another object of the present invention is to provide a device for measuring crack opening displacement which is simple in construction and easy to build yet is rugged and capable of precision performance.
Another object of the present invention is to provide a device for measuring crack opening displacement wherein the device is characterized by a simple means for securing the device in the crack opening.
Still another object of the present invention is to provide a crack opening displacement gage that has an improved symmetrical differential capacitive displacement sensor operating on the basis of plate area changes rather than changes in distance between capacitor plates.